Diverting water changes a waterways natural flow
When natural seasonal flow variations are interrupted by diverting water extreme care must be taken to maintain the amount of water needed to support healthy ecosystems. The amount of water needed is called environmental flow, which considers maximum and minimum flow levels to support a healthy ecosystem. Failure to provide an environmental flow can have serious consequences for water quality and mahinga kai.
Impacts of diverting water on water quality and mahinga kai:
- Modified flow - flow changes as stream banks are modified and realigned, which can lead to changes in the benthic (bottom) structure of the stream/river bed, coarse substrates such as gravels and boulders are replaced and covered by sand and silt. Changes in water levels and flow variability also alters available mahinga kai habitat and the invertebrates they feed on.
- Modified channel form - erosion from vegetation removal along banks and changes to stream flow after construction of a road crossing or similar barrier can lead to scouring and breakdown of stream and river banks, impacting on mahinga kai habitat.
- Damage to banks and floodplains - varying flows and flash floods threaten the stability of a river bank, increasing its vulnerability at times of flooding and damaging breeding and feeding habitat for mahinga kai.
- Reduce habitat - a decrease in water levels reduces habitat for fish and can impact feeding and spawning success.
- Reduce specialist habitats - a decrease in water levels reduces flow to riparian wetlands, backwaters and intermittent streams.
- Decrease species abundance and diversity - aquatic species have developed life history strategies in direct response to natural flows, for example, diadromous fish species migrate up and down the river at various times of the year and rely on preferred velocities and depths.
- Change sediment accumulation - flow reduction effects movement and deposition of sediments in streams and rivers.
- Change water quality parameters - turbidity, temperature and oxygen levels can increase with reduced flows in rivers.
- Increase algae accumulation - algae respond to changes in temperature and nutrients which are likely to increase with reduction of flow especially during summer months.
- Increased nutrients - a decrease in flow may increase the concentration of nutrients within a river.